The manufacture of cylinder heads for internal combustion engines poses difficult manufacturing problems. The cylinder head of an internal combustion engine, whether for a spark driven gasoline internal combustion engine or a compression ignition diesel engine is a complex article of manufacture with many requirements. A cylinder head generally closes the engine cylinders and contains the many fuel explosions that drive the internal combustion engine, provides separate passageways for the air intake to the cylinders and for the engine exhaust, carries the multiplicity of valves needed to control the air intake and engine exhaust, provides a separate passageway for coolant to remove heat from the cylinder head, and provides separate passageways for fuel injectors and the means to operate the fuel injectors.
The walls forming the complex passageways and cavities of a cylinder head must withstand the extreme internal pressures, temperatures and temperature variations generated by the operation of an internal combustion engine, and must be particularly strong in compression-ignition diesel engines. On the other hand, it is desirable that the internal walls of the cylinder head, particularly those walls between coolant passageways and the cylinder closures, permit the effective transfer of heat from the cylinder head, and it is also important that the cylinder head include minimal metal to reduce its weight and cost.
These countervailing requirements make the manufacture of reliable cylinder heads difficult. Furthermore, these complex parts are manufactured by the thousands and assembled into vehicles that must operate reliably under an extreme variety of conditions. The manufacture of reliable cylinder heads is particularly important because of the high cost of their replacement. Consequently, the manufacture of cylinder heads has been the subject of the developmental efforts of engine and automobile manufacturers throughout the world for years.
Cylinder heads are most generally manufactured by casting them from iron alloys. The casting of the cylinder head portion that closes the cylinders, carries the intake and exhaust valves and fuel injectors and provides the passageways for the air intake, exhaust and coolant requires a mold carrying a plurality of core elements. To provide effective cooling of the cylinder head and effective air intake and exhaust from the cylinders of the internal combustion engine, the passageways for the air intake and exhaust are best interlaced with the coolant passageways within the cylinder head portion. The cavities for coolant, air intake and exhaust must, of course, be formed by core elements within the mold that can be removed when the casting metal solidifies.
In prior casting methods where a one-piece coolant jacket core has been used, a plurality of core elements, to form each of the separate passageways for the exhaust and for the air intake, have been manually set into the "green sand" of the mold by workmen. The individual placement by workmen of the core elements forming the intake and exhaust passageways of the cylinder head is necessary in order to interlace the plurality of such core elements with the one-piece coolant jacket core. In this method, the "green sand" of the mold is provided with preformed cavities to position and hold each of the plurality of separate mold elements that are to form the exhaust passageways and air intake passageways in the cast cylinder head. The "green sand" is a mixture of sand, clay and water which has been pressure-formed into the mold element. Although such green sand provides sufficient structural integrity to contain the molten metal during casting and to form the exterior walls of the casting, it provides no great structural integrity, easily yielding to the pressure that may be exerted by the hands of workmen. Thus, in this manufacturing method, the green sand mold is easily deformed by the workmen in placing any one or more of the plurality of core elements forming the intake and exhaust passageways of the cylinder head in a green sand mold element. The green sand mold is thus incapable of providing and maintaining a reliable location of the plurality of core elements. As the result of such casting methods, there is no assurance that the thickness of the internal walls of the cylinder head will be reliably maintained during manufacture, and there is a substantial risk that unreliable castings will result.
In prior casting methods where a one-piece core formed the plurality of passageways for the air intake to the cylinders and a one-piece core formed the plurality of exhaust passageways from the plurality of cylinders, the coolant passageways are formed with two core elements to permit the interlacing of the portions of the cores forming the air intake passageways and the exhaust passageways with the two core element portions forming the passageways for coolant. In such manufacturing methods, a first element of the coolant core is placed in the green sand mold, and the cores forming the passageways for the air intake and for the engine exhaust are then placed in the green sand mold. The second element of the coolant core is then attached by an adhesive to the first part of the coolant jacket core. This method necessarily requires the use of an adhesive that can be easily spread on the coolant jacket core elements, that will set within the shortest possible time, that will hold the two parts of the coolant jacket core element together as one piece and maintain their position during the casting process, and that may be removed from the casting after the casting metal solidifies. This method results in substantial costs and opportunities for unreliable castings. It is necessary that workmen apply the adhesive correctly so that the adhesive reliably maintains the coolant jacket core elements together during casting. It is also necessary that the workmen reliably assemble the two elements of the coolant jacket core during manufacture. Furthermore, this process requires time for applying the adhesive, assembling the coolant jacket core elements together and allowing the adhesive to set before the mold can be used for casting, and it introduces into the mold an unnecessary foreign element in the form of the adhesive and a potentially unreliable interface between the two elements of the coolant jacket core.
In the casting process, the formation of elongated, narrow, open cavities has not been possible without supporting a long core element forming the elongated open cavity at intervals of several inches throughout the length of the cavity. For example, core elements on the order of 20"-22" in length and about 1" in diameter, cannot be used to form such cavities without a plurality of supports that extend from the core element to adjacent walls of the mold or core and are spaced along the length of the core element between the core element and adjacent walls of the mold assembly. Such long unsupported core elements, because they are less dense than the casting metal and are unsupported, tend to be displaced as the molten metal fills the mold cavities and frequently to fail, for example, by fracturing. Where such long core elements have been used, it has been necessary for the workmen in the factory to place small supporting metal elements, called "chaplets" in the casting art, between such long core elements and the adjoining walls of the mold. Such chaplets prevent the displacement of the long core element as the cavity of the mold fills with molten metal and prevent failure of the long core element, for example, by breaking due to the force imposed upon the core element by the molten metal. The metal chaplets, however, remain in the walls of the casting that form the long open cavity. The metal chaplets are provided with a metallic coating that is intended to fuse with the casting metal at the interface between the chaplet and the casting wall; however, the hands of the workmen placing chaplets into the mold frequently became dirty because of their work in casting operations, and it is practically impossible to keep the surface of the chaplets free of contaminants that interfere with the fusion between the chaplets and the casting walls. Thus, small passageways and other discontinuities in the casting wall can be formed at the interface between such chaplets and the casting metal that makes up the wall for the casting. For many engine manufacturers the most significant warranty expense of an internal combustion engine results from failures and unreliability due to the use of chaplets in supporting core elements within a mold for an internal combustion engine.
Because of the complexity of the cylinder head, past cylinder heads have included more than one part. In addition to the portion of the cylinder head assembly that closes the cylinders, provides the intake, exhaust and coolant passageways, and carries the intake and exhaust valves and fuel injectors, such cylinder head assemblies have included separate castings for the intake manifold and fuel rail. The manufacture of such cylinder head assemblies requires machining of the cylinder head casting, the intake manifold casting and the fuel rail casting to provide sealing surfaces for gaskets, and the labor of their assembly. Such cylinder head assemblies have further possibilities of unreliability because of improper assembly, gasket failure and the like, and impose upon the manufacturer and their dealers a requirement for separate parts inventories.
The aggregate unnecessary costs of such prior casting methods, in the manufacture of the thousands of cylinder heads and in the repair and maintenance of such cylinder head assemblies during their life, is inestimable.